: This is a revised application for a new R01 from an established investigator. The PI has requested 3 years of support to investigate the genetic basis of hyperhomocysteinemia and its relationship to vascular pathology in several inbred strains of mice. Levels of Hcy in the C57BL/6J strain are twofold higher than the A/J and C3H/HeJ strains; (C57BL/6J x A/J) F1 animals are similar (but not identical) to the A/J parent; by contrast, (C57BL/6J x C3H/HeJ)F1 animals exhibit so-called heterosis in that their levels are apparently higher than either parent. In Specific Aim I, the underlying genetic basis of these differences will be investigated by determining Hcy levels in recombinant inbred strains for each of the two pairs, C57BL/6J x C3H/HeJ, and by analyzing 320 progeny from an F2 intercross for QTLs. Hcy levels will also be examined in 21 consomic strains (congenic for each of 21 A/J chromes) in the C57BL/6J background. In Specific Aim IV, the potential relationship of increased Hcy levels to vascular pathology will be investigated by measuring lipid-containing intimal lesions in large and medium sized arteries from the RI strains and from PL/J (see below). In humans, decreased activity of methylene tetrahydrofolate reductase (MTHFR) contributes to elevated Hcy levels because its product is a cofactor for conversion of Hcy to Methionine. The PI has discovered that the PL/J strain exhibits approximately twofold lower activity of MTHFR than most inbred strains, and, in Specific Aim II, proposes to investigate the genetic basis for this difference bet determining MTHFR activity in 200 progeny from an (C57BL/6J x PL/J)F2 intercross. Finally, in Specific Aim III, the effect of age, sex, pregnancy, and folate supplementation on Hcy levels will be examined in C57BL/6J and A/J MICE, and in C57BL/6J mice that carry one of two mutations, Gli3 or ApoB, that apparently cause a secondary increase in Hcy levels by an unknown mechanism. A part of these experiments, the PI will also complete a survey of Hcy levels in MTHFR activity in additional inbred strains.